Down’s syndrome

What is Down’s syndrome?

Down’s syndrome is caused by the presence of an extra copy of chromosome 21.

What causes Down’s syndrome?

Normally, each cell in the human body contains 23 pairs of chromosomes, which contain the genetic material that determines all our inherited characteristics. We receive half of each chromosome pair from our mother and the other half from our father. Individuals with the most common form of Down’s syndrome, trisomy 21, have an extra chromosome 21.

No one knows exactly why this chromosomal condition occurs, but it does appear to be related to the age of the mother. The older a woman is when she gives birth, the higher the risk of her baby having Down’s syndrome. At 45, the risk is one in 30.

Children with Down’s syndrome are often identified at birth as a result of the physical characteristics associated with the syndrome. Some children exhibit only a few characteristics; others exhibit many. Because some of these features are also seen in people without Down’s syndrome, genetic testing must be done to confirm diagnosis. The most common features associated with Down’s syndrome include:

Down's syndrome diagnosis and treatment: How do I know if my child has Down’s syndrome?

The results of these tests, together with the woman’s age, are used to predict the risk of Down’s syndrome, which becomes increasingly common as women get older. If this suggests a high risk, more diagnostic tests must be done to confirm the diagnosis. Screening may sometimes suggest a high risk when in fact Down’s syndrome is not present (a “false positive” result), and sometimes show a low risk when the foetus does have Down’s syndrome (a “false negative” result).Ultrasound screening for Down’s syndrome is done with a special type of called nuchal translucency. This focuses on measuring the space between the spine and the back of the baby’s neck. Babies with Down’s syndrome usually have more fluid in their neck than normal.

The antenatal tests used to diagnose Down’s syndrome are chorionic villus sampling (CVS) after 10 weeks of pregnancy, or amniocentesis after 15 weeks of pregnancy. These tests do carry a small risk of miscarriage because they require the insertion of a needle into the fluid surrounding the foetus or into the placenta in order to examine the baby’s chromosomes.

These tests accurately diagnose Down’s syndrome 98% to 99% of the time.

After a baby is born, physical characteristics may lead doctors to suspect Down’s syndrome. A genetic test called a karyotype must be done to confirm diagnosis. A karyotype involves testing a small amount of the baby’s blood.

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